Molten zinc coated steel sheets are used in various fields, mainly for automobile body applications because they exhibit excellent weldability and paintability. Molten zinc coated steel sheets are used for such applications after being press formed. However, molten zinc coated steel sheets have a drawback. They are inferior to cold-rolled steel sheets in terms of press formability. This is because molten coated steel sheets have larger sliding resistance against press dies than cold-rolled steel sheets. That is, molten zinc coated steel sheets do not easily enter press dies at bead portions having high sliding resistance against the dies, causing rupture of the steel sheets.
For example, an alloyed molten zinc coated steel sheet is manufactured by coating a steel sheet with zinc, and subsequently heating the steel sheet to diffuse Fe in the steel sheet and Zn in the coated layer to each other to effect an alloying reaction, thereby forming an Fe—Zn alloy phase. The Fe—Zn alloy phase is generally a film constituted by a Γ phase, a δ1 phase, and a ζ phase. As the Fe concentration in the film decreases, that is, in the order of the Γ phase, the δ1 phase, and the ζ phase, the hardness and the melting point tend to decrease. In view of slidability, the film preferably has a high Fe concentration, which provides high hardness, a high melting point, and less probability of causing adhesion. Thus, alloyed molten zinc coated steel sheets that are intended to have high press formability are manufactured to have high average Fe concentrations in their films.
However, films with high Fe concentrations tend to have a Γ phase, which is hard and brittle, at the interfaces between coated layers and steel sheets. This tends to cause a phenomenon called “powdering” that the films come off from the interfaces while the steel sheets are processed, which has been a problem.
In view of those problems, we performed thorough studies which resulted in the findings disclosed in Japanese Unexamined Patent Application Publication No. 2003-306781. A flat portion on a surface of an alloyed molten zinc coated steel sheet protrudes from the surrounding areas. The flat portion is specifically brought into contact with a press die when the steel sheet is press formed. Hence, reduction of sliding resistance of the flat portion provides improved press formability. The sliding resistance of the flat portion is reduced by preventing adhesion of the coated layer to a die. This is achieved by forming a hard film with a high melting point on the surface of the coated layer.
We thus performed additional studies and found that control of the thickness of an oxide film on the surface layer of the flat portion is effective, and such control of the thickness of an oxide film on the surface layer of the flat portion prevents adhesion of the coated layer to a die and provides good slidability. We further found that such an oxide film is effectively formed by a method of bringing a coated surface layer into contact with an acid solution to form a Zn-based oxide layer thereon.
We then discovered a technique of bringing an alloyed molten zinc coated steel sheet into contact with an acid solution to form an oxide mainly containing Zn (hereinafter, referred to as a “Zn-based oxide”) on a surface of the steel sheet, thereby suppressing adhesion of the coated layer to a press die and enhancing slidability. That technique, disclosed in Japanese Patent No. 3608519 is directed to a method for manufacturing a steel sheet in which water vapor with a temperature of 100° C. or more is sprayed on a steel sheet to which an acid treatment solution has been applied. The method was intended to form an oxide film on a surface of the steel sheet in a short time with reliability.
However, in that method, it is difficult to spray water vapor evenly over a surface of a steel sheet. Hence, water vapor is actually sprayed unevenly over a surface of a steel sheet. This generates unevenness on the surface of the steel sheet and degrades the appearance of the surface.
It could therefore be helpful to provide an apparatus for manufacturing a molten zinc coated steel sheet, the apparatus being capable of stably forming a necessary oxide film without degrading the appearance of the surface of a steel sheet, the apparatus being easily put into practical use.